Rangaraju, V. | Search bioRxiv

bioRxiv2023

VAP spatially stabilizes dendritic mitochondria to locally fuel synaptic plasticity

Synapses are pivotal sites of memory formation and undergo plasticity in response to external inputs. Consequently, synapses are hotspots of energy consumption and are susceptible to dysfunction when their energy supplies are perturbed. Mitochondria are stabilized near synapses via cytoskeletal tethering and serve as local energy supplies to fuel synaptic plasticity. However, the mechanisms that tether and stabilize neuronal mitochondria for long durations and determine the spatial dendritic segment supported during synaptic plasticity are unknown. We identified a list of novel mitochondrial-cytoskeletal interactors in neurons using APEX-based proximity labeling. We narrowed down the protein candidates that exclusively tether mitochondria to actin near postsynaptic spines using high-resolution Airyscan confocal imaging. We find that VAP, the vesicle-associated membrane protein-associated protein implicated in Amyotrophic Lateral Sclerosis and interacts with the endoplasmic reticulum, stabilizes mitochondria via actin near the spines. To test if the VAP-dependent stable mitochondrial compartments can locally support synaptic plasticity, we investigated individual spines stimulated by two-photon glutamate uncaging for spine plasticity induction and their adjacent spines. We find that, along with actin, VAP functions as a spatial stabilizer of mitochondrial compartments to sustain synaptic plasticity for up to ~60 min and as a spatial ruler that determines the ~30 m length of the dendritic segment supporting synaptic plasticity.

neuroscience↗

bioRxiv2022

NMNAT2 is the major NAD+ provider for vesicular glycolysis generating on-board energy for fast axonal transport cargos

BackgroundBioenergetic maladaptations and axonopathy are often found in the early stages of neurodegeneration. Nicotinamide adenine dinucleotide (NAD), an essential cofactor for energy metabolism, is mainly synthesized by Nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2) in CNS neurons. NMNAT2 mRNA levels are reduced in the brains of Alzheimers, Parkinsons and Huntingtons disease. Here we addressed whether NMNAT2 is required for axonal health of cortical glutamatergic neurons, whose far-projecting axons are vulnerable to neurodegenerative conditions. We also tested if NMNAT2 maintains axonal health by ensuring proper axonal ATP levels for axonal transport, a critical function of axons. MethodsWe generated mouse and cultured neuron models to determine the impact of NMNAT2 loss from cortical glutamatergic neurons on axonal transport, energetic metabolism, and morphological integrity. In addition, we determined if exogenous NAD supplementation or inhibiting NAD hydrolase sterile alpha and TIR motif-containing protein 1 (SARM1) prevented axonal deficits caused by NMNAT2 loss. Our study used a combination of genetic, molecular biology, immunohistochemistry, biochemistry, fluorescent time-lapse imaging, live imaging with optical sensors, and anti-sense oligos application. ResultsWe provide in vivo evidence that NMNAT2 in cortical glutamatergic neurons is required for axonal survival. Using in vivo and in vitro studies we demonstrate that NMNAT2 protects axons by ensuring the proper NAD-redox potential in distal axons of cortical neurons to support glycolysis on vesicular cargos, thus ensuring "onboard" ATP production fueling axonal transport. Exogenous NAD+ supplementation to NMNAT2 KO cortical neurons restores glycolysis and resumes fast axonal transport. Finally, we demonstrate both in vitro and in vivo that reducing the activity of SARM1, an NAD degradation enzyme, can reduce axonal transport deficits and suppress axon degeneration in NMNAT2 KO neurons. ConclusionNMNAT2 ensures axonal health by maintaining NAD redox potential in distal axons to ensure efficient vesicular glycolysis required for fast axonal transport.

neuroscience↗